Telecommunication channels, such as but not limited to metallic (copper) wirelines, used for the transport of digital subscriber loop service, are sometimes supported by one or more back-up or standby channels, through which an interruption in service due to a failure or fault (e.g., an excessive number of errored seconds) in an active channel can be restored. For efficient use of the available wireline channels, the usual practice is to have a plurality of main channels backed-up by a single standby channel. In order to avoid having to immediately dispatch service personnel to investigate the faulty channel and manually switch in the standby circuit, it is customary practice to employ a protection switching scheme, which is automatically operated by a supervisory fault-monitoring mechanism, to substitute the back-up channel for the faulty channel. Moreover, to ensure that the protected data channel is properly routed at both ends of the replaced transport path, signaling for controlling the operation of the protection switch must occur between each end of the circuit. One way to accomplish this is to employ some form of in-band coding. Unfortunately, such an approach is relatively slow and requires a special processing function to decode the signaling.
Some data channels carry an embedded overhead channel, that may be used to send performance monitoring messages or alarm signaling between termination equipment. As a non-limiting example, a DS1 data channel framed with extended super frame (ESF) framing has a 4 kbps overhead channel, known as facility data link (FDL). This auxiliary channel provides a relatively fast and robust mechanism for conveying protection switch signaling information between the circuit termination equipments, to control operation of the protection switch for substituting the standby channel for the faulty channel (or switching the main channel back in service upon clearing of the fault). Unfortunately, because ESF framed data does not necessarily exist in the payload data carried by main and standby channels being transported over currently deployed digital telecommunication (e.g., DS1) equipment, it cannot be relied upon to provide protection switching signaling.